This invention relates to the recovery of hydrocarbons from oil shale. More particularly, this invention relates to a process for providing substantially complete removal of recoverable hydrocarbons from oil shale retorts by plugging channels that bypass unretorted shale.
Oil shale refers to deposits of sedimentary rocks containing an organic material called kerogen from which hydrocarbons can be recovered by heat treatment. Shale oil commonly includes the liquid hydrocarbon product recovered upon the heating of oil shale to sufficient temperature to decompose kerogen, forming liquid shale oil hydrocarbons and coke residue. Oil (Kerogen-bearing) shale can be found throughout the world and in particular in Colorado, Utah, and Wyoming.
Commonly, in in situ retorting the shale is heated with hot retorting gas to decompose the kerogen. Since naturally occurring oil shale deposits are substantially impermeable to gases, the shale cannot be heated easily by hot retorting gases. Commonly, prior to retorting, the shale is cracked or broken in such a way that the shale becomes permeable to retorting gases. Many methods including hydraulic fracturing, mining, electrical heating, explosive rubblizing etc. have been proposed to provide permeability and porosity in oil shale retorts. For example a retort or retorting zone containing rubblized shale can be formed beneath the ground, preferably wholly within an oil shale zone by using conventional mining techniques to remove a portion of the shale within a retorting area, preferably at the bottom of the zone. This mined area is called a void space. The balance of the shale is then rubblized, and the rubblized shale is expanded into the void space. The rubblizing provides a permeable, porous shale mass filling the zone. Explosives can be used to rubblize the shale, preferably forming a volume having rubble with uniform particle size wholly enclosed by surrounding solid unrubblized rock walls. M. Prats et al. Soluble-Salt Process For In Situ Recovery of Hydrocarbons From Oil Shale, Journal of Petroleum Technology, September 1977, pages 1078-1088, teaches the use of water to provide permeability in in situ oil shale retorts. The water, by dissolving soluble salt such as sodium carbonate and sodium bicarbonate can introduce very high permeability in the shale as the leached pore volume is large and connected. However, Prats also teaches that fine shale particles produced by the leaching action can reduce the injectability of water causing low permeability. Since this article deals with the production of permeability there is no teaching of the use of fluids to plug depleted zones or channels in retorts.
After the underground retort zone containing rubblized porous shale is formed, hot gases are passed through the rubblized shale to effectively heat and decompose the kerogen and in some cases to aid in the removal of liquid hydrocarbon product. Commonly a gas such as air or air mixed with steam or a fuel such as hydrocarbons is passed through the rubblized zone while igniting the retort to heat the shale. Most commonly, an oxygen containing gas or air is pumped into one end of the retort to support the burning of a portion of the kerogen or the shale forming a flame front throughout an entire end of the shale retort. The flame front passes slowly through the rubblized porous shale body producing hot gasses used to heat the kerogen, resulting in liquid products and coke residue in the rubble. The liquid product can be collected in various places in the retort.
The flame front should proceed uniformly throughout the rubblized oil shale evenly heating the entire zone to insure maximum recovery. In this way, the maximum amount of liquid hydrocarbons would be recovered and little unheated, unretorted shale would remain in the retort. In practice, however, the uniform passage of the flame front through the rubblized shale is an ideal not ordinarily achieved. Since it is impossible to produce rubblized shale with constant particle size, an even distribution of particle sizes, an even distribution of bulk densities, and without void spaces, the flame front cannot pass uniformly through the rubblized shale. One or more portions of the flame front can proceed more rapidly through certain portions of the less dense rubblized shale than through other portions. As one or more portions of the flame front move more rapidly through the retort, a zone or channel of shale which has been retorted and depleted of liquid hydrocarbons can be formed entirely from one end of the retort to another providing a low resistance path through the retort for the retorting gases. This depleted zone or channel causes the retorting gases to bypass a substantial portion of the unretorted shale that is somewhat more dense than the depleted zones. Since these depleted zones or channels provide a relatively low resistance path for the retorting gases, the unretorted shale would remain bypassed, unretorted and unheated, unless the channels or depleted zones are plugged, resulting in the loss of a substantial amount of hydrocarbon.
Willman U.S. Pat. No. 3,198,249 teaches injecting a bank of dissolved solids into a burned out area to plug the porous formation. Willman teaches that the solid residue left by the bank acts to plug void spaces in the formation. Willman suffers the disadvantage that substantial expense would be incurred in injecting sufficient solids into a formation to result in substantial plugging. Further the solids would tend to plug the entire formation.
Thus a need exists to provide a process that insures the production of the maximum amount of recoverable, extractable hydrocarbons from in situ retorts by plugging or sealing depleted zones or channels which occur during the retorting of oil shale.